Mailing List Archive

Cedar McKay wrote: ... > this is wildly off topic, but if you have worse output with mythtv than > tivo, you have something wrong. Check your encoding prefs, and also your > tv-out solution (hardware and configuration). If you want to continue > the discussion, it should be moved to myth users.

Cedar, I appreciate you enthusiasm on this in your nine points earlier (actually ten but you used "3" twice ;-). However, comparing your picture with the picture at your friend's house introduces several variable. If you are going to suggest that everyone should see MythTV's picture as having better quality, I'd like to point out a few limitations.

Jitter: If you watch the crawler on CNN, it is smooth on commercial products but has nervous twitches on myth. The frame buffer should be updated during vertical synch but myth doesn't have access to this timing. Further, the frame buffer is updated about every 1/30 of a second but because the update is subject to a multitasking scheduler, the timing is subject to when the process get the CPU. The "jitter reduction" option can improve the timing but if the update straddles the vertical refresh, there will be two updates during one display frame and no update on the next causing this twitch. You'll often see several seconds of smooth crawler followed by a few seconds of twitches. Dedicated hardware can know the refresh timing and not be subject to a process scheduler.

Deinterlace: Again using a crawler, when myth is paused the characters look smooth but are jagged in motion. On commercial products the characters are smooth in motion but have 'stair steps' when paused. I believe this is because they are doing something smart about updating the two interlaced fields so that scan lines are updated like the original interlaced signal. Myth has a frame of both fields updated once per full frame. The default linear blend deinterlace code waters down the data so it is neither a true reproduction of the interlace fields nor is it as crisp as the original sampling.

Color reproduction: In a side by side comparison, my myth picture doesn't look as life-like because the color and intensity ranges are distorted by the bttv device while recording. One would hope that the default contrast and color settings would be 'line level' but apparently they are not. Brightness and hue are close to neutral but contrast and color at 50% are clearly distorted on each bttv card I've tried. For contrast, white and anything near white it is fully saturated while medium intensities are dull. For color, when solids are bright but not over-driven, medium tones look washed out.

Setting the v4l contrast to something below 40% (usually 36-38%) greatly improves the picture. Color has greater variation from one card to the next but is usually best between 25-40%. Even after making these adjustments and adjusting the TV set to match, the color and intensity is not as close to the direct TV image as the commercial products.

While there is nothing wrong with the story that you like your picture more than your friend's, it is unrealistic at this point for me to expect MythTV to have a better quality picture than the custom hardware of the retail DVRs.

Bruce Markey wrote > Cedar McKay wrote: > ... > > this is wildly off topic, but if you have worse output with mythtv than > > tivo, you have something wrong. Check your encoding prefs, and also your > > tv-out solution (hardware and configuration). If you want to continue > > the discussion, it should be moved to myth users. > > Cedar, I appreciate you enthusiasm on this in your nine > points earlier (actually ten but you used "3" twice ;-). > However, comparing your picture with the picture at your > friend's house introduces several variable. If you are > going to suggest that everyone should see MythTV's picture > as having better quality, I'd like to point out a few > limitations.

I also want to add in that you can also get wildly different signal qualities due to cabling. I was getting a very poor picture for low channels (below 16) at first, but was able to improve them all by trying a different cable between my tuner card and the wall. I would also think that all cable leading from the cable provider to everyone's home is not equal (though, I'm sure they strive to do so).

Also, I should add that the overall picture quality (despite the jitter) on my MythTV setup is better than my TiVo picture and that's because I run my TiVo at "Basic" quality to get the most storage possible. My MythTV box happens to have a drive with similar space as my TiVO. I get hardly any artifacts when recording on MythTV at a 1Gig/hour quality level, even with fast movement, where the same content on the TiVo would look extremely blocky. I'm sure this is because of the MPEG4 codec settings I'm using are that much better than the MPEG2 codec settings used by the TiVo for "Basic" quality, but the highest quality on the TiVo looks superb for all fast-motion content.